1. Field of the Invention
The present invention relates to a light source unit including a plurality of light-emitting elements, which are arranged in line.
2. Description of the Related Art
A light source unit comprising a light-emitting element array is known. In this array, a plurality of light-emitting elements (LED, for instance) are aligned (see Japanese Patent Laid-Open Publication No. 6-225080). For example, the light-emitting element array is constituted such that the light-emitting elements are mounted side by side on a printed-circuit board on which a circuit pattern to be connected to a power source is printed. Electrodes of the respective light-emitting elements are electrically connected to the circuit pattern by wire bonding.
This kind of the light source unit comes into use as an original-illuminating light source of a scanner and a copying machine instead of a linear fluorescent lamp conventionally used, since even illuminance distribution is obtained in a longitudinal direction of the light-emitting element array. When this light source unit is used as the original-illuminating light source, a width direction of an original (scanning direction) is set so as to coincide with the longitudinal direction of the light-emitting element array.
Further, it is considered to use the above-mentioned light source unit as a light source for optically fixing a thermosensitive recording paper having optical fixation properties. In this case, higher illuminance is required in comparison with the case of the original-illuminating light source. Thus, the light-emitting element having higher illuminance is used. At the same time, the light-emitting element array is designed such that both ends thereof are provided with reflection planes extending in the longitudinal direction. Meanwhile, when pursuing high power, a calorific value becomes large so that measures for heating are required in accordance therewith.
In the conventional light-emitting element array, the respective light-emitting elements and the circuit pattern to be connected thereto by wire bonding are mounted on a single printed-circuit board. Although resin is generally used as a material of the printed-circuit board, heat conductance of the resin is lower than that of metal. In view of this, it is considered to use a printed-circuit board, which is made of metal instead of resin. The metal is aluminum and so forth. However, unit cost of the aluminum board is expensive in comparison with the resinous board. In addition, processing cost of etching and so forth is high. Thus, there arises a problem in that the cost increases.
The circuit pattern is disposed at each side of the light-emitting elements in the longitudinal direction of the light-emitting element array. Consequently, the circuit pattern is positioned between the light-emitting element and the reflection plane. Thus, it is impossible to shorten an interval between the light-emitting element and the reflection plane. When the interval is broad, there arises a problem in that unevenness of intensity distribution is enlarged in the longitudinal direction (scanning direction). This problem is caused because the light-emitting element array is merely an aggregation of point sources (light-emitting elements) from a microscopic view, although this array is macroscopically regarded as a linear light source. Therefore, the unevenness of the intensity distribution is somewhat caused in the scanning direction in accordance with arrangement intervals of the respective light-emitting elements. The unevenness of the intensity distribution is enlarged and emphasized as the interval between the light-emitting element and the reflection plane becomes broader.